ARTIFICIAL DIELECTRIC ISOLATOR FOR THz RADIATION

ABSTRACT

An isolator based on a waveguide-based artificial dielectric medium is scalable to a range of desired terahertz frequencies, has non-reciprocal transmission and provides low insertion loss and high isolation at various tunable terahertz frequencies, far exceeding the performance of other terahertz isolators, and rivaling that of commercial optical isolators based on the Faraday effect. Because terahertz artificial dielectrics are low loss, inexpensive, and easy to fabricate, this approach offers a promising new route for polarization control of free-space terahertz beams in various instrumentation applications. Artificial dielectrics are man-made media that mimic properties of naturally occurring dielectric media, or even manifest properties that cannot generally occur in nature. A simple and effective strategy implements a polarizing-beam-splitter and a quarter wave plate to form a highly effective isolator. Performance of the device is believed to exceed that of any other experimentally demonstrated method for isolation of back-reflections for terahertz beams.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. National phase application Ser.No. 16/494,221 filed Sep. 13, 2019, which is a national phase filingunder 35 U.S.C. § 371 of International Application No.PCT/US/2018/022158 filed Mar. 13, 2018, which claims priority to U.S.Provisional Patent Application No. 62/470,685 filed Mar. 13, 2017, andU.S. Provisional Patent Application No. 62/474,301 filed Mar. 21, 2017,the entire contents of which are hereby incorporated by referenceherein.

GOVERNMENT SUPPORT

This invention was made with government support under grant 1609521awarded by the National Science Foundation. The government has certainrights in the invention.

This application is addressed to an isolator construction based onartificial dielectric materials for operation in the terahertz spectralregime. Applicants hereby incorporate by reference herein in itsentirety the full text, drawings and disclosure of provisionalapplication Ser. No. 62/432,157 filed Dec. 9, 2016 in the U.S. Patentand Trademark Office, as well as the full text, drawings and disclosureof provisional application Ser. No. 62/470,685 filed Mar. 13, 2017 inthe U.S. Patent and Trademark Office, and also the full text, drawingsand disclosure with its appendix of provisional application Ser. No.62/474,301 filed Mar. 21, 2017 in the U.S. Patent and Trademark Office,which compactly describe a THz domain isolator construction and a seriesof measurements evaluating its operation with terahertz signals from afrequency-matched Polarizing Beam Splitter.

BACKGROUND AND DETAILED DESCRIPTION

A polarizing-beam-splitter (PBS) is a device that splits a linearlypolarized beam into two orthogonal polarization components, in apre-determined power ratio. In the THz region, there have been a fewstudies on PBSs using metamaterials [1], dielectric bi-layers [2],diffraction gratings [3], and recently, using form birefringence [4].(numbers in square brackets refer to publications listed in thebibliography, infra. In all of these prior art cases, the fabrication ofthe device is complicated and not readily scalable. Here we presentexperimental characterization of a PBS involving a far simpler geometryand exhibiting remarkable performance. Our design is based on artificialdielectrics [5, 6], man-made media that mimic properties of naturallyoccurring dielectric media, or even manifest properties that cannotgenerally occur in nature. Although originally introduced by themicrowave community, the wavelength scaling that result when shiftingfrom microwaves to THz waves gives new life to this waveguide-basedtechnology [7]. At the design frequency of 0.2 THz, our PBS exhibits anextinction ratio of 42 dB in transmission and 28 dB in reflection withan overall insertion loss of 0.18 dB, the best values ever reported inthe THz range. Further, by combining our PBS with a quarter-wave-platebased on the same artificial-dielectric technology, we demonstrate a THzisolator with an isolation of 52 dB and an insertion loss less than onedB, at a frequency of 0.46 THz. This isolation is more than three ordersof magnitude higher than recently demonstrated THz isolators based ongraphene [8], and the insertion loss is considerably lower thanpreviously demonstrated THz isolators based on ferrite materials [9,10]. In addition, our design does not require an externally appliedmagnetic field. Indeed, the performance of our device rivals that ofcommercially available Faraday isolators at optical wavelengths. Thissimple method for achieving very high isolation will be invaluable fornumerous applications involving high-power THz sources [11] or THzsystems with highly sensitive receivers [12].

The isolator of the present invention will be described in connectionwith exemplary embodiments wherein a polarization beam splitter (PBS)provides polarization selectivity in the beam path, and a quarter waveplate (QWP) provides a phase delay, which in combination with the PBS,is used to reject reflections of the beam. The PBS operates withfrequencies below cutoff, which depends upon the spacing of metal plateswhich constitute the artificial dielectric and the incident angle, whilethe QWP operates above a cutoff frequency that depends upon the platespacing and introduces a phase delay which depends upon the plate widthand the plate spacing. These two elements constructed with artificialdielectric materials result in superior operating characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be understood from the description and drawingsherein, taken together with the claims appended hereto, wherein

FIGS. 1(A)-1(D) show a polarizing beam splitter (PBS) fabricated withartificial dielectic construction and simulations. Illustrated detailsinclude 1(A) Photograph of the PBS device with close-up views showing asquare section of the stack-of-plates (looking on axis) and thebottom-corner of the mounting post. 1(B) Geometry of a singlestainless-steel plate. (c, d) FEM simulations of the beam propagation ata frequency of 0.2 THz when the input electric field is linearlypolarized 1(C) perpendicular, and 1(D) parallel, to the plates. The beamdiameter is 1 cm and the angle of incidence is 45°. All dimensions arein millimeters.

FIG. 2 schematically illustrates the experimental setup, where “Tx” isthe abbreviation for transmitter” and “Rx” is the abbreviation for“receiver.” The plane of the paper corresponds to the horizontal plane,which is also the plane of the plates. Gray areas represent thepropagating beam. The complete detector sub-system (shown within theblack dashed enclosure) can be moved intact, from the on-axis positionto the 90°-off-axis position to measure either transmission orreflection. All the polyethylene lenses are in confocal configurationsto achieve maximum power transfer through the optical system. The insetdiagram illustrates the general propagation behavior of the device (notdrawn to scale and exaggerated for clarity) for an input beam with anarbitrary linear electric-field polarization direction and a broadspectrum having frequencies extending above and below the TE₁-mode'scutoff.

FIGS. 3(A)-3(C) show transmission and reflection spectra. 3(A)Transmission spectra when the input polarization is parallel to theplates. The blue curve is the reference with no device, the green curveis with the device at normal incidence, and the red curve is with thedevice at 45° incidence. The device spectra correspond to TE₁-modepropagation through the device. The sharp dips at 0.56 THz and 0.75 THzare due to water-vapor absorption. 3(B) Reflection spectra when theinput polarization is parallel to the plates. The red curve is with thedevice at 45° incidence and the blue curve is the reference with apolished aluminum mirror replacing the device. 3(C) Transmission spectrawhen the input polarization is perpendicular to the plates. The bluecurve is the reference with no device and the red curve is with thedevice at 45° incidence. The device spectrum corresponds to TEM-modepropagation through the device. The insets in 3(B) and 3(C) show thecorresponding time-domain signals.

FIGS. 4(A)-4(B) show the power efficiency and extinction ratio. 4(A)Power efficiency for the transmission (blue dots) and reflection (reddots) arms, within the operational bandwidth. The blue solid curve givesthe theoretical efficiency for the transmission arm taking into accountonly the ohmic loss. 4(B) Cross-polarization extinction ratio for thetransmission (blue dots) and reflection (red dots) arms, within theoperational bandwidth.

FIG. 5 is a schematic of the experimental setup used to investigate theIsolator. The isolator consists of the PBS combined with a QWP as shownby the red dashed enclosure in the upper right corner of the Figure. Aphotographic view of the isolator is shown inset. The input polarizationis vertical and perpendicular to the plane of the paper. A gold mirror Mwas used to create the back-reflection, and a silicon beam splitter SBSwas used to tap it off to the receiver. The PBS is at 45° incidence, bydesign, and the QWP is at a 12° incidence to eliminate theback-reflections originating from it.

FIG. 6 is a schematic diagram of the isolator with the respectivepolarization directions given in operation (perpendicular to the planeof the paper or vertical, or parallel to the plane of the paper orhorizontal).

FIG. 7 shows measured isolation curves. Results are shown for twodifferent quarter wave plates, QWP₁ (blue curve) and QWP₂ (red curve)designs, with close-up views of the peaks shown as insets.

DETAILED DESCRIPTION—DESIGN AND FABRICATION

The artificial dielectric medium as employed herein for splittingpolarizations consists of a uniform stack of identical, rectangularmetal plates. This stack-of-plates is electromagnetically equivalent toa stacked array of parallel-plate waveguides (PPWGs) [7], and in anexemplary embodiment are made of 30 μm thick stainless steel and arespaced 300 μm apart, as seen in the prototype device shown in FIG. 1(A).The plates and the spacers are fabricated by chemical etching to avoidany strain or burring, which helps to maintain their flatness. Thedevice is assembled by stacking the plates and spacers intermittentlyalong two locating posts, such that the plates are free-standing,supported only by their ends. At each end, there is a square pad with amounting hole as shown in FIG. 1(B), which gives the actual shape of aplate. Once assembled, this stacked-plate arrangement results in a clearaperture of 20 mm. The magnified, close-up view of the 4 mm squaresection of the clear aperture illustrates the flatness of the plates andthe uniformity of their spacing. Taking a slice across this image andusing a graphical reconstruction method, we measure an averagecenter-to-center plate spacing of 330 μm, with a standard deviation of 5μm.

In the designed device geometry, the THz beam is directed at thestack-of-plates at an angle of 45° to the virtual surface emulated bythe plate edges, with the plane of incidence parallel to the platesurfaces. The operation of the PBS relies on both the TEM and TE₁fundamental modes of the PPWG [13, 14]. When the input electric-field islinearly polarized perpendicular to the plates, only TEM modes areexcited in the PPWG array, and the beam propagates through the devicewithout altering its path. This behavior is illustrated in the FEMsimulation result shown in FIG. 1(C), which plots the instantaneouselectric field of the propagating beam along the axial cross-sectionparallel to the plate surfaces, at a frequency of 0.2 THz. As long asthe input beam diameter is sufficiently larger than the plate spacing(for proper mode-matching) and the interaction path-length is short,this TEM-mode propagation will be a very efficient (i.e., low loss)process [13]. On the other hand, when the input electric-field islinearly polarized parallel to the plates, only TE₁ modes can be excitedin the PPWG array, and the propagation is governed by the mode's cutofffrequency. Input frequencies that are above the cutoff will propagatethrough the device, while those that are below the cutoff will bereflected. In fact, these below-cutoff frequencies will be totally andspecularly reflected in a well-defined beam [7]. This behavior isillustrated in the FEM simulation result shown in FIG. 1(D), which plotsthe instantaneous magnetic field of the propagating beam at a frequencyof 0.2 THz.

For oblique incidence, the TE₁-mode cutoff frequency is given by c/(2bcos α), where c is the free-space velocity, b is the plate spacing, andα is the incidence angle [15]. For the demonstrated device, the cutoffis at 0.7 THz when the device is illuminated at 45°. Now, if the inputelectric-field is linearly polarized at an arbitrary angle (between 0°and 90°) to the plates, both the TEM and TE₁ modes are excitedsimultaneously. Then, the portion of the input beam (the perpendicularcomponent) propagating via the TEM mode exits the device on axis,polarized perpendicular to the plates. This TEM-mode contribution isindependent of the frequency. In contrast, the portion that could excitethe TE₁ mode (the parallel component), if below cutoff, is totallyreflected at 90° to the input axis, polarized parallel to the plates. Byvarying the angle of the input polarization, one can control the powerdivision into the two output arms, thereby realizing a versatile PBS.Incidentally, if there is any parallel component at a frequency abovethe cutoff, this portion would propagate through the device via the TE₁mode and exit the device with a slight lateral shift, polarized parallelto the plates. This general behavior of the device for an incident beamwith an arbitrary linear polarization direction having frequenciesextending below and above the cutoff is schematically illustrated in theinset of FIG. 2 . Since the PBS operation would be hampered by anyexcitation of the TE₁ mode, the upper limit of the operational bandwidthof the PBS is set by the mode's cutoff frequency. Therefore, it followsthat the bandwidth can be increased by decreasing the plate spacingand/or increasing the incidence angle.

Experimental Characterization—PBS

A prototype PBS device was experimentally investigated in transmissionand reflection configurations using a THz time-domain spectroscopysystem, as schematically depicted in FIG. 2 . In this spectroscopysystem, both the transmitter and receiver modules are fiber coupled tothe main controller unit, so as to accommodate the multiple polarizationaxes and spatial configurations. Throughout the experiment, the devicewas located between two wire-grid polarizers to purify the input anddetected linear polarizations. Via external optics, the input beam wasformed to a frequency-independent 1/e Gaussian diameter of approximately10 mm and fairly well collimated. The same optical arrangement wasemployed for the output beam to maintain input-output symmetry. Whilethe input optics were fixed in space, the detector sub-system could bemoved (intact) from the on-axis position to the 90°-off-axis position tochange from a transmission configuration to a reflection configuration.A 16 mm diameter aperture was situated in close proximity to the inputtransverse-plane of the device. This eliminated any energy “spill-over”,providing a true reference signal when the device was not in the beampath, and also served as a marker for the beam axis. In addition tothree-axis linear translation, the device mount also included aprecision rotation stage to adjust the incidence angle in the horizontalplane, along with precision control of the tilt in two perpendicularvertical planes, allowing complete three-axis rotational positioning.

FIGS. 3(A)-3(C) illustrate various measured amplitude spectra that wereobtained by Fourier transforming the detected time-domain signals. FIG.3(A) shows spectra corresponding to the purely TE₁-mode behavior of thedevice in transmission. During this measurement, both the transmitterand receiver polarization axes (along with the input and outputpolarizer axes) were kept horizontal. Here, the blue curve correspondsto the reference signal when there is no device in the path of the beam.The sharp dips seen at 0.56 THz and 0.75 THz are due to water vaporabsorption. The green curve corresponds to the signal when the device isin the path of the beam at normal incidence. This spectrum indicates acutoff near 0.5 THz, which is expected for a 300 μm plate spacing. Thered curve corresponds to the signal when the device is at 45° incidence(the designed operating configuration), and as predicted by theory, thecutoff shifts to a value near 0.7 THz. This TE₁-mode diagnosticmeasurement is indicative of the quality of the device and serves as ademonstration of the operational bandwidth of the PBS.

For the spectra in FIG. 3(B), the polarization axes of the transmitter,receiver, and the polarizers were maintained horizontal as before, butthe detector sub-system was moved to measure the reflected signal. Thered curve corresponds to the reflected signal when the device is at 45°incidence. The blue curve corresponds to the reference signal when thedevice is replaced by a polished aluminum mirror. The inset gives thedetected time-domain signals, and along with the spectra, they prove thehighly efficient and non-dispersive broad-band operation of the devicein reflection. The high-frequency attenuation of the device spectrumwhich appears to build up starting close to 0.7 THz is consistent withthe TE₁-mode transmission spectrum in FIG. 3(A). Since this attenuationmanifests for relatively low amplitude levels of the input spectrum (asevident by the reference), there is only minimal change in the reflectedtemporal signal.

For the spectra in FIG. 3(C), the measurement configuration was changedback to transmission, and the polarization axes of the transmitter,receiver, and the polarizers, were rotated to be vertical. Therefore,this configuration investigates the purely TEM-mode behavior of thedevice. The blue curve corresponds to the reference signal when there isno device in the beam path. The red curve corresponds to the signal whenthe device is at 45° incidence to the input beam. The detectedtime-domain signals are given in the inset, and as before, along withthe amplitude spectra, prove the highly efficient and non-dispersivebroad-band operation of the device in transmission. This observation isnot surprising since the TEM mode of the PPWG is a very low-loss anddispersion-less propagating mode [13]. However, it should be noted thatin order to obtain this efficient propagation it was important for thebeam axis to be aligned so as to be parallel to the plate surfaces withhigh accuracy, and also for the input polarization direction to beexceptionally well perpendicular to the plate surfaces. Deviations fromthese two conditions would result in additional losses, as the obliqueincidence results in a longer interaction path-length, compared to thatwith normal incidence. It is not only the added ohmic loss that comesinto play, but also the relative parallelism of the stack of plates.

Using the spectra in FIGS. 3(B) and (C), one can deduce the powerefficiency of the device for the transmission and reflection arms. Theseefficiency curves are plotted in FIG. 4(A) by the blue dots and red dotsfor transmission and reflection, respectively, within the operationalbandwidth of the PBS. For the reflection arm, the efficiency curve isrelatively flat throughout the bandwidth, and indicates a powerefficiency of 96% at both 0.2 THz and 0.5 THz, for example. Thiscorresponds to an insertion loss of only 0.18 dB. For the transmissionarm, the efficiency is 96% at 0.2 THz, and drops to 84% at 0.5 THz. Thiscorresponds to an insertion loss of 0.76 dB. For comparison, alsoplotted is the theoretical transmission (blue solid curve) taking intoaccount only the ohmic loss associated with TEM-mode propagation. Thediscrepancy with the experimental curve (especially as the frequencyincreases) implies that there are other sources of loss. Part of thisextra loss is caused by the non-negligible impedance mismatch at theinput and output surfaces of the device, even in the case of TEM-modepropagation [16]. This gives rise to two small reflections from thesesurfaces, which may also be affected by the finite thickness of theplates. In fact, these reflections played a role in the subsequentmeasurements that were carried out to estimate the cross-polarizationextinction ratios of the PBS.

In the next measurement, the input polarization was oriented at 45° tothe horizontal plate surfaces, and both the vertical and horizontalcomponents of the output were measured, for both the transmission andreflection configurations separately. Therefore, for the transmissionarm, in addition to the major component of the output that is polarizedperpendicular to the plates, this also measures the minor component thatis polarized parallel to the plates. This minor cross-polarizationcomponent is a result of energy leakage due to subtle deviceimperfections. The squared ratio of these two components gives theextinction ratio, which is plotted by the blue dots in FIG. 4(B). Thiscurve indicates extinction ratios of 42 dB and 39 dB at 0.2 THz and 0.5THz, respectively. Similarly, for the reflection arm, in addition to themajor component polarized parallel to the plates, this measures theminor component polarized perpendicular to the plates. In this case, thecross-polarization component is due to the two TEM-mode reflections atthe input and output surfaces, as discussed above. The estimatedextinction ratio is plotted by the red dots in FIG. 4(B), where theobserved ripple is due to the associated Fabry-Perot effect. This curveindicates extinction ratios of 28 dB and 22 dB at 0.2 THz and 0.5 THz,respectively. These values are not as impressive as for the transmissionarm; however, a simple way to improve this extinction would be to add apolarizer to the reflection arm. This polarizer could be an identicalartificial-dielectric device as used for the PBS, and would be extremelyefficient since the beam would now be at normal incidence.

Experimental Characterization—Isolator

We constructed an isolator for the THz region using a metal plateartificial dielectric construction, and the isolator performancecharacteristics were then measured. In optics, the primary purpose of anisolator is to minimize or eliminate feedback (back-reflections), whiletransmitting sufficient power in the forward direction. Isolators areessential for the stable and reliable operation of lasers, especiallyhigh-power ones, in well-aligned complex optical systems, whereback-reflections are inevitable. High-contrast isolators are alsocritical components in full-duplex communication systems [17]. In theTHz region, there have been a few experimental studies on isolators, forexample, using ferrite materials [see: D. H. Martin and R. J. Wylde,“Wideband circulators for use at frequencies above 100 GHz to beyond 350GHz,” IEEE Trans. Microwave Theory Tech. 57, 99 (2009); and M. Shalaby,M. Peccianti, Y. Ozturk, and R. Morandotti, “A magnetic non-reciprocalisolator for broadband terahertz operation,” Nature Commun. 4, 1558(2013)] or using graphene [M. Tamagnone, C. Moldovan, J.-M. Poumirol, A.B. Kuzmenko, A. M. Ionescu, J. R. Mosig, and J. Perruisseau-Carrier,“Near optimal graphene terahertz non-reciprocal isolator,” NatureCommun. 7, 11216 (2016).]. Our THz isolator is designed by combining apolarizing beam splitter (PBS) with a quarter-wave plate (QWP) that isalso fabricated utilizing the same artificial-dielectric technology.This isolator design is shown schematically by the dashed-line enclosurein the upper right corner of the schematic diagram given in FIG. 5 , andthis experimental configuration was used to investigate the isolatorbehavior. A detailed schematic diagram of the isolator with therespective polarization directions is given in FIG. 6 . Here, ⊥ denotesperpendicular to the plane of the paper (or vertical) and H denotesparallel to the plane of the paper (or horizontal).

The QWP was fabricated using the same 30 μm thick stainless-steel platesas shown, for example in PCT/US17/65524 (claiming priority to 62/432,157Filed Dec. 9, 2016) which describes a Polarizing beam splitter for THzradiation, but with a plate spacing of 1 mm in a first design (denotedQWP₁), and a spacing of 0.8 mm in second design (denoted QWP₂). For theproper operation of the QWP, the plane of the plates is oriented at 45°to the input vertical polarization set by the PBS. Then, half of theinput energy will propagate via the TEM mode and the other half via theTE₁ mode. After propagating through the device at different velocities,these two orthogonal polarization components will acquire a relativephase of 90° at a certain frequency. It can be shown that this occurs ata frequency given by 0.5cd[(1/b)²+(0.5/d)²], where d is thepropagation-path length. Accordingly, since d=2 mm, this should occur at0.32 THz and 0.49 THz for QWP₁ and QWP₂, respectively. Now, when thereis a back-reflection, due to the “double-pass” through the QWP, thephase difference will become 180°. This will effectively rotate thepolarization axis of the resultant reflected beam by 90° [18], renderingit horizontal, and be diverted in the off-axis direction by the PBS,essentially isolating it from the input-beam path.

As shown in FIG. 6 , a gold minor M was used to create the backreflection, and it was detected by tapping it off using a silicon beamsplitter, with and without the isolator in place. The ratio of these twospectra, after converting to decibels, gives the isolation curves, asplotted in FIG. 7 . The QWP was mounted with an azimuthal tilt of 12° toeliminate the back-reflections generated at its surfaces. (This tilt isseen in both the schematic and the photo shown inset in FIG. 6 .) Forthe QWP₁ design, the maximum isolation is 48 dB, and this occurs at 0.32THz, exactly as the theory predicts. For the QWP₂ design, the maximumisolation is 52 dB, and this occurs at 0.46 THz, slightly shifted fromthe theoretical value. This discrepancy may be due to a weakertightening of the plates assembly, resulting in a slightly larger thanexpected plate separation. For completeness, the forward powertransmission of the isolator was also measured in a differenttransmission configuration, and it was 85% and 80% (equivalent to aninsertion loss of 0.71 dB and 0.97 dB) for the QWP₁ and QWP₂ designs,respectively, at the peak isolation frequencies. A suitable arrangementthat may provide continuous control of the plate spacing would allowdynamic tunability of the isolation-peak, adding versatility to theisolator.

Discussion

In conclusion, we have experimentally demonstrated a highly efficientand versatile PBS for the THz spectral regime based on artificialdielectrics. The device geometry is exceedingly simple compared to allprevious PBS attempts for this spectral region. Furthermore, the PBSexhibits insertion losses as low as 0.18 dB and cross-polarizationextinction ratios as high as 42 dB. By combining this PBS with a QWP ofthe same artificial-dielectric technology, we also demonstrate a THzisolator with peak isolations as high as 52 dB. This work opens up awhole new generation of highly effective, highly efficient, easy tofabricate, and inexpensive artificial-dielectric polarimetric devicesfor the THz region. Furthermore, since the devices are made from stackedmetallic plates, as opposed to conventional dielectric materials, theywill also uniquely possess extremely high power handling capabilities,limited only by the breakdown of air within the plates.

BIBLIOGRAPHY

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1-7. (canceled)
 8. A system comprising: a THz device for a radar system;a send/receive full duplex communication system or some other high powerapplication system that operates with THz-frequency signals; wherein theTHz device includes a first array of spaced-apart conductive platesforming an artificial-dielectric polarizing beam splitter (PBS), and asecond array of spaced-apart conductive plates forming anartificial-dielectric quarter wave plate (QWP) in the path of a beamfrom the first array, the second array being positioned and oriented toremove any reflections or back-scatter emanating from it.
 9. The systemof claim 8 wherein the artificial-dielectric quarter wave plate (QWP) isabsent any periodic perforations.
 10. The system of claim 8 wherein theartificial-dielectric quarter wave plate (QWP) provides a phase delay.11. The system of claim 8 wherein the artificial-dielectric quarter waveplate (QWP) is fabricated using 30 μm thick stainless-steel plates. 12.The system of claim 8 wherein the polarization beam splitter (PBS)provides polarization selectivity in the beam path, and the quarter waveplate (QWP) provides a phase delay, which in combination with the PBS,is used to reject reflections of the beam.